Currently two animal protocols (one rabbit and one mouse) are ongoing. The focus of the rabbit studies is to understand and prevent immunologic rejection of the transplanted limbal stem cells using donor tissue manipulation and local immunosuppression including sustained release implants. Studies currently in progress involve tissue engineering using cultured corneal epithelial cells for the treatment stem cell deficiency. These latter studies will likely provide the basis for future clinical studies using cultured corneal epithelial cells. The mouse studies will be focused on limbal stem cell biology and immunlogy taking advantage of various transgenic strains. Studies will also attempt to determine the long-term survival of transplantated limbal stem cells and the effect of systemic immunosuppression using mouse anti-IL2 receptor antibody. The plasticity of limbal stem cells in various stages of development will also be studied. Using gene expression studies, attempts will be made to determine the factors that are important for maintaining "stemness". Human studies evaluating the continued presence of donor limbal stem cells is being done in parallel with the animal studies. In these studies limbal corneal rims from patients who underwent previous stem cell allogeneic limbal stem cell transplants and then a penetrating keratoplasty have been analysed. The presence or absence of donor stem cells is being evaluated using microsatellite techniques. Initial results have confirmed the validity of the technique being employed and donor stem cells can remain present for years after transplant. These findings will have therapeutic implications since patients receiving allogeneic transplants must be immunosuppressed.